As an example of a cutting insert (hereinafter referred to as “insert” in some cases) for cutting a workpiece, Japanese Unexamined Patent Publication No. 2004-284010 discloses a cutter insert used for a milling tool. This publication discloses that when the cutter insert is attached to a tool body, a positive axial rake angle can easily be achieved by multiple twisting a rectangular parallelepiped shaped basic form, and also discloses that a cutting edge of the cutter insert is curved in a linear or balloon shape.
However, because in the cutter insert of Japanese Unexamined Patent Publication No. 2004-284010, the axial rake angle is positive, the end portion of a peripheral surface cutting edge closer to the end surface of a cutting tool body, namely, the portions corresponding to corner parts in a planar view are firstly contacted against the workpiece during the cutting process. Hence, there is a risk of damage to the end portion of the peripheral surface cutting edge under cutting conditions that cause a large contact impact against the workpiece, such as high feed rate cutting.